1. Field of the Invention
The present disclosure relates to a display device, and particularly, to a display device having a heat radiation sheet capable of easily radiating heat in a small-sized liquid crystal display device (LCD).
2. Discussion of the Related Art
Recently, a flat panel display device such as a liquid crystal display device (LCD) is used as a display device not only in various portable electronic devices such as mobile phones, notebook computers and tablet computers, but also in large-sized electronic devices such as televisions.
Such an LCD is a transmittance-type display device, which displays an image by adjusting light transmittance of liquid crystal having dielectric anisotropy using an electric field. To this end, an LCD includes a backlight which is a light source that generates light penetrating a liquid crystal layer for an image display.
In general, the backlight is classified into two types, a side type backlight in which a lamp is disposed at a side surface of a liquid crystal panel and provides light to the liquid crystal layer, and a downright type backlight in which a lamp directly provides light from a lower side of a liquid crystal panel.
The side type backlight is disposed at a side surface of the liquid crystal panel and may provide light to a liquid crystal layer through a reflective plate and a light guiding plate. Since it is possible to obtain a thin display device, such a side type backlight is mainly used in a notebook computer which requires a thin display device. However, since a lamp that generates light is disposed at a side surface of the liquid crystal panel in the side type backlight, it is difficult for the side type backlight to be applied to large-sized electronic devices and also it is difficult to obtain high brightness because light is supplied via the light guiding plate. Thus, such a side type backlight is not appropriate to be used in a large-sized LCD TV which has recently been highlighted.
The downright type backlight is being applied not only to a large-sized liquid crystal panel as light generated from a lamp is directly supplied to a liquid crystal layer, but to a liquid crystal panel for manufacturing an LCD TV owing to its high brightness.
Meanwhile, as a lamp for backlight, a light source such as a light emitting diode (LED) which is one of self-luminous devices, is mainly used rather than a fluorescent lamp. Since such an LED emits monochromatic light, i.e., R, G and B, there are advantages in that color reproducibility is good when the LED is applied to a backlight and a driving power can be reduced.
FIG. 1 is a cross-sectional view illustrating a structure of a liquid crystal display device provided with a backlight having an LED according to the related art.
As shown in FIG. 1, an LCD includes a liquid crystal panel 10 which includes a first substrate 1, a second substrate 2, and a liquid crystal layer (not shown) disposed between the first substrate 1 and the second substrate 2, and is configured to implement an image as a signal is applied thereto from outside, an LED substrate 32 disposed at a lower side surface of the liquid crystal panel 10 and having thereon a plurality of LEDs 34 that emit light, a light guiding plate 35 disposed at a lower side of the liquid crystal panel 10 and configured to guide light emitted from the LEDs 34 and provide the light to the liquid crystal panel 10, an optical sheet 38 disposed between the liquid crystal panel 10 and the light guiding plate 35 and including a diffusion sheet 38a and prism sheets 38b and 38c that diffuse and condense light guided from the light guiding plate 35 and provided to the liquid crystal panel 10, a reflective plate 36 disposed below the light guiding plate 35 and configured to reflect light guided to a lower side of the light guiding plate 35, a lower cover 40 in which the reflective plate 36, the light guiding plate 35, the optical sheet 38, and the LED substrate 32 are accommodated, a guide panel 42 coupled with the lower cover 40 and in which the reflective plate 36, the light guiding plate 35, the optical sheet 38, and the LED substrate 32 are mounted and on which the liquid crystal panel 10 is positioned, and an upper cover 46 coupled with the guide panel 42 and configured to mount the liquid crystal panel 10 thereunder.
The first substrate 1 of the liquid crystal panel 10 is an array substrate on which a thin film transistor is formed. On the first substrate 1, not only the thin film transistor, but various wirings and pixel electrodes are formed. The second substrate 2 is a color filter substrate and a color filter and a black matrix are formed thereon.
The lower cover 40 is configured to assemble a backlight including the reflective plate 36, the light guiding plate 35, the optical sheet 38, and the LEDs 34, and its wall is extended upwardly from a bottom so that parts of the backlight may be positioned for assembly. The upper cover 46 is coupled with the guide panel 42 and the lower cover 40 in order to mount the liquid crystal panel 10 and the backlight therein.
In such a related art liquid crystal display device, the LED substrate 32 is disposed on at least one side surface of the light guiding plate 35 and a plurality of LEDs 34 are disposed on the LED substrate 32 so that light emitted from the LEDs 34 is made to be incident to a side surface, that is, a light incident surface of the light guiding plate 35 and then provided to the liquid crystal panel 10.
Meanwhile, an LED is known as a display device that generates more heat than a fluorescent lamp. Thus, when LEDs 34 are used as a light source of a backlight, a temperature of a liquid crystal display device rises, compared with a case where a fluorescent lamp is used, so that a lifespan of the LEDs 34 is decreased, the light guiding plate and the optical sheet are deteriorated, or a liquid crystal layer of the liquid crystal panel 10 is deteriorated, resulting in faulty products. To this end, when the LEDs 34 are used as a light source of the backlight, it is necessary to prevent temperature rise in a liquid crystal display device by promptly discharging heat generated from the LEDs 34 to outside.
In the related art liquid crystal display device as shown in FIG. 1, the LED substrate 32 includes a metal plate with a set area, and a flexible printed circuit (FPC) (not shown) made of a soft film is attached to an end of the LED substrate 32 which includes a metal so that the LEDs 34 are electrically connected to an external power source. Further, the lower cover 40, the guide panel 42, and the upper cover 46 include a metal having high heat conductivity.
Thus, in such a related art liquid crystal display device, heat generated from the LEDs 34 is discharged through the LED substrate 32, the lower cover 40, the guide panel 42, and the upper cover 46 so that temperature rise in a liquid crystal device can be prevented.
However, the liquid crystal display device having such structure shown in FIG. 1 is a large-sized one which is mainly used in an electronic device such as a TV or a notebook computer, and a small-sized display device used in a portable device such as a mobile phone has a different structure from that of the display device of FIG. 1.
FIG. 2 is a cross-sectional view illustrating a small-sized liquid crystal display device in accordance with the related art. The structure of the small-sized liquid crystal display device and that of the large-sized liquid crystal display device of FIG. 2 are similar to each other, but there is a difference in an assembling process of the LEDs 34 and the LED substrate 32. Thus descriptions on the same elements having the same structure will be omitted for clarity purpose.
Here, in the liquid crystal display devices of FIG. 1 and FIG. 2, shapes of the guide panels are different from each other, but may be manufactured in various forms and shapes depending on a size of a liquid crystal panel and an assembly of other parts. Thus other shapes thereof will be omitted for clarity purpose. Further, in the small-sized liquid crystal display device of FIG. 2, an upper cover is removed, but may be assembled if necessary.
As shown in FIG. 2, a plurality of LEDs 34 are disposed at a side surface of a light guiding plate 35 to face a light incident surface of the light guiding plate 35. Here, the plurality of LEDs 34 are mounted on the LED substrate 32. In the large-sized liquid crystal display device as shown in FIG. 1, the LED substrate 32 is disposed between the light guiding plate 35 and the guide panel 42 so that a mounting surface of the LED substrate 32 is in contact with the lower cover 40 or the guide panel 42. On the contrary, in the small-sized liquid crystal display device as shown in FIG. 2, the LED substrate 32 is disposed on an upper surface of the light guiding plate 35 and the guide panel 42 so that a mounting surface of the LED substrate 32 faces a lower direction and only the LED 34 is disposed between the light guiding plate 35 and the guide panel 42.
The reason why the LED substrate 32 is disposed on an upper surface of the light guiding plate 35 and the guide panel 42 is in order to minimize a size of the liquid crystal display device. Since such a small-sized liquid crystal display device is mainly used in communication devices, it is required to minimize a size and a weight of the device. Thus, the liquid crystal display device is manufactured in such a manner shown in FIG. 2 in order to minimize a size of the liquid crystal display device, compared with a large-sized liquid crystal display device.
Accordingly, in a large-sized liquid crystal display device shown in FIG. 1, since a rear surface of the LED substrate 32 having a relatively large area contacts the lower cover 40 or the guide panel 42, in a case where heat is generated from the LED 32, the heat is conducted to the lower cover 40 or the guide panel 42 through the LED substrate 32 to thus be emitted outside. On the contrary, in the small-sized liquid crystal display device shown in FIG. 2, since part of a mounting surface of the LED substrate 32 contacts an upper surface of the guide panel 42, heat discharge efficiency is decreased.
Moreover, in a large-sized liquid crystal display device shown in FIG. 1, the LED substrate 32 including a metal is used, whereas in a small-sized liquid crystal display device shown in FIG. 2, since the LEDs 34 are directly mounted on an FPC which has low heat conduction efficiency, heat generated from the LEDs 34 cannot be substantially discharged to outside through the FPC and the guide panel 42. This may cause a problem of temperature rise in the LED 34 of the small-sized liquid crystal display device cannot be prevented.